1. Field of the Invention
This invention relates to a method for the electrical protection of a liner used for sealing the stern tube shaft of a ship.
2. Description of the Prior Art
Heretofore, a so-called oil bath system such as the one shown in FIG. 1, has frequently been used as a stern tube shaft seal device. In this shaft seal device, the forward side end face of a boss of a propeller 2 mounted on a propeller shaft 1 is connected to the flange face of a liner 3 fitted to the propeller shaft 1 by means of bolts. A plurality of sealing rings 7 fixed by a casing including a cover ring 4, intermediate rings 5 and a flange ring 6 are slidably fitted to the outer periphery of the liner 3 in order to seal the shaft. This shaft seal device is included to prevent intrusion of sea water into the ship and is a very important feature for safe navigation. Using this device, since the liner is always in sliding contact with the sealing rings during navigation, it is necessary for the liner to have excellent wear resistance and excellent corrosion resistance to sea water. Accordingly, a high chromium stainless steel is frequently used as the liner-constituting material.
Since the stainless steel which is used as the liner-constituting material inherently has a good corrosion resistance, no anti-corrosive treatment has been necessary in the past.
However, because of the fairly recent increase in pollution of sea water, even stainless steel liners presently have a tendency to suffer such corrosion phenomena as gap corrosion and porous corrosion. Furthermore, this corrosion is continually increasing in severity. In practice, these forms of corrosion occur in that portion of the liner having sliding contact with the sealing rings on the sea water side. Difficulties such as oil leakage and damage to the sealing ring rubber frequently result.
Accordingly, when such corrosion occurs on a liner, it must be disassembled from the propeller shaft and reprocessed. If the corrosion is extreme, a fresh liner must be mounted and the corroded liner is discarded. In view of the docking cost required by the exchange or refurbishment of liners and the cost of the maintenance itself, a great economic loss is suffered. Hence, it would be most desirable to develop an effective method for protection of liners from corrosion in sea water.
In order to electrically protect the shell of a hull or steel sheet pipes from corrosion, a galvanic anode method has been generally used successfully in the past. In the technique protection is accomplished by using sacrificial anodes made of aluminum and/or zinc. Typical of such devices are those disclosed by the following references: U.S. Pat. No. 3,623,968 (Bohne) which discloses an easily installable sacrificial anode of cylindrical or tubular form to be applied at the joints of welded sections of underground coated pipe to which it is electrically connected; U.S. Pat. No. 3,274,085 to Rutemitter et al; which discloses a consumable aluminum galvanic anodes for cathodic protection of ship ballast tanks among other things; DT-OS 1,446,351 to Maurin et al; which discloses a corrosion protection device for metallic surfaces arranged beneath the earth or submerged in water; and DT-PS 1,133,962 to Determann which discloses the corrosion protection of all submerged parts of the stern of a ship. Other patents related in subject matter are: U.S. Pat. No. 3,562,124 to Leon et al; U.S. Pat. No. 3,616,419 to Bagnulo; U.S. Pat. No. 3,721,618 to Reding et al; U.S. Pat. No. 3,723,282 to Pashak; U.S. Pat. No. 3,864,234 to Wasson; and DT-OS 2,012,864 to Meisel-Krone. However, such devices have not been used in the past in conjunction with stern tube sealing liners.